1. Field of the Invention
The present invention relates to a device for transmitting an alternating analog signal from a sensor to a measurement room remote from the sensor, comprising a measuring preamplifier located in the vicinity of the sensor for providing a preamplified signal from the analog signal of the sensor, a transmission line having two conductors for conveying the preamplified signal to the measurement room, an electronic measuring circuit located in the measurement room and receiving the transmitted preamplified signal for providing an output signal representing the signal from the sensor, and a power supply circuit located in the measurement room and comprising a constant DC current generator connected to supply terminals of the measuring preamplifier through the transmission line.
A sensor such as a piezoelectric quartz gives a low level alternating analog signal and preamplification is required if the transmission line is of a length possibly reaching a few kilometers.
To overcome the problem of attenuation which the resistance of the line would cause in a voltage transmission, current transmission is often used as shown in FIG. 1 of the accompanying drawings.
2. Description of the Prior Art
In FIG. 1, there is shown at 10 a piezoelectric type sensor connected by a cable 11, preferably of the screened type, to the input of a preamplifier 20. The preamplifier is here of the electric charge amplifier type, with a feedback capacitor 21, so as to be adapted to the nature of the signal from sensor 10.
The preamplifier 20 is situated at an offset location in the vicinity of the sensor 10, in an area which may be difficult of access as in the case of a nuclear power station.
There is shown at 30 an electronic circuit transforming the preamplified signal. Here, this signal, which is an alternating voltage, is transformed into an alternating current. The circuit 30 may be formed by a resistance 31 in series with the collector-emitter path of a transistor 32 having its base terminal 33 connected to the output 22 of the preamplifier 20. The endmost terminals 34 and 35 of the transistor 32 and resistance 31 assembly are respectively connected to the input terminals 41-42 of a transmission line 40.
The transmission line 40 conveys the preamplified signal, in current form, to a measurement room represented by a frame 1 of dash-dot lines.
An electronic measuring circuit 50 receiving the transmitted signal is formed here symbolically by a load resistance 51, a capacitor 52 letting through the alternating component of the signal and an indicator 53 giving the value of the output signal U.about..
A power supply circuit comprises a constant DC current generator 60, located in the measurement room, and connected to the output terminals 43-44 of transmission line 40.
Supply terminals 23-24 of the preamplifier 20 are furthermore connected to the input terminals 41-42 of line 40, so that the preamplifier can be supplied with power from the measurement room 1 and through the transmission line 40. An intermediate reference voltage may be created by means of a divider not shown.
It can be seen that capacitor 52 serves to prevent the supply DC current from passing into indicator 53. The preamplifier 20 must also have a low and constant power consumption in relation to the current representing the transmitted signal.
It is often required to be able to check the proper operation of the transmission chain going from sensor 10 to indicator 53 of the measurement circuit. To this end, it is known to insert an alternating voltage source 70 between sensor 10 and the input of preamplifier 20, for example by means of a test switch 71.
If the offset location where the preamplifier 20 is to be found is not accessible and does not have an alternating voltage source, there may be inserted between the sensor 10 and the input of the preamplifier 20 a transformer secondary whose primary is supplied with an alternating voltage from the measurement room. But, in this case, an additional transmission line is required.
In order not to affect the output signal U.about. by the checking test, it is known to use in the electronic measuring circuit 50 a differential amplifier (not shown in FIG. 1), receiving on the one hand, the output voltage of the measuring circuit 50 and, on the other hand, the voltage of the alternating source 70 (when the latter is located in the measurement room).
To prevent the noise appearing in the transmission with preamplified signals which are too weak, or to prevent saturation appearing with signals which are too strong, it is often desirable to be able to modify the gain of preamplifier 20. To this end, it is known to use a gain modification circuit 80, comprising in the case of FIG. 1 an additional feedback capacitor 81 which can be connected in parallel with feedback capacitor 21 through switch means 82.
If the offset location where the preamplifier 20 is to be found is not accessible, the switch means 82 may be a relay controlled from the measurement room. But, in this case, yet another additional line must be provided.
It can be noted that the distributed capacitance of line 40 influences the result of the measurement for a part of the measuring current is diverted by said capacitance, and this all the more so since the load resistance 51, of a not negligible value, imposes a non-zero voltage at the output terminals 43-44 of line 40.
On the other hand, to avoid voltages being induced in line 40 by other conductors through which high currents flow and which are disposed in the vicinity of line 40, it is desirable for said line to be of the screened type, and so more expensive. But, in this case, the distributed capacitance of the line is increased, which aggravates the previous shortcoming.